DNA sequencing is driving genomics research and discovery. The completion of the Human Genome Project was a monumental achievement involving an incredible amount of combined efforts among genome centers and scientists worldwide. This decade-long project was completed using the Sanger sequencing method, which remains the staple genome sequencing methodology in high-throughput genome sequencing centers. The main reason behind the prolonged success of this method was its basic and efficient, yet elegant, method of dideoxy chain termination. With incremental improvements in Sanger sequencing—including the use of laser-induced fluorescent excitation of energy transfer dyes, engineered DNA polymerases, capillary electrophoresis, sample preparation, informatics, and sequence analysis software—the Sanger sequencing platform has been able to maintain its status. Current state-of-the-art Sanger based DNA sequencers can produce over 700 bases of clearly readable sequence in a single run from templates up to 30 kb in length. However, as with most technological inventions, the continual improvements in this sequencing platform have come to a stagnant plateau, with the current cost estimate for producing a high-quality microbial genome draft sequence at around $10,000 per megabasepair. Current DNA sequencers based on the Sanger method allow up to 384 samples to be analyzed in parallel.
It is evident that exploiting the complete human genome sequence for clinical medicine and health care requires accurate low-cost and high-throughput DNA sequencing methods. Indeed, both the public (National Human Genome Research Institute, NHGRI) and private genomic sciences sectors (The J. Craig Venter Science Foundation and Archon X prize for genomics) have issued a call for the development of next-generation sequencing technology that will reduce the cost of sequencing to one-ten thousandth of its current cost over the next ten years. Accordingly, to overcome the limitations of current conventional sequencing technologies, a variety of new DNA sequencing methods have been investigated, including sequencing-by-synthesis (SBS) approaches such as pyrosequencing (Ronaghi et al. (1998) Science 281: 363-365), sequencing of single DNA molecules (Braslaysky et al. (2003) Proc. Natl. Acad. Sci. USA 100: 3960-3964), and polymerase colonies (“polony” sequencing) (Mitra et al. (2003) Anal. Biochem. 320: 55-65).
The concept of DNA sequencing-by-synthesis (SBS) was revealed in 1988 with an attempt to sequence DNA by detecting the pyrophosphate group that is generated when a nucleotide is incorporated in a DNA polymerase reaction (Hyman (1999) Anal. Biochem. 174: 423-436). Subsequent SBS technologies were based on additional ways to detect the incorporation of a nucleotide to a growing DNA strand. In general, conventional SBS uses an oligonucleotide primer designed to anneal to a predetermined position of the sample template molecule to be sequenced. The primer-template complex is presented with a nucleotide in the presence of a polymerase enzyme. If the nucleotide is complementary to the position on the sample template molecule that is directly 3′ of the end of the oligonucleotide primer, then the DNA polymerase will extend the primer with the nucleotide. The incorporation of the nucleotide and the identity of the inserted nucleotide can then be detected by, e.g., the emission of light, a change in fluorescence, a change in pH (see, e.g., U.S. Pat. No. 7,932,034), a change in enzyme conformation, or some other physical or chemical change in the reaction (see, e.g., WO 1993/023564 and WO 1989/009283; Seo et al. (2005) “Four-color DNA sequencing by synthesis on a chip using photocleavable fluorescent nucleotides,” PNAS 102: 5926-59). Upon each successful incorporation of a nucleotide, a signal is detected that reflects the occurrence, identity, and number of nucleotide incorporations. Unincorporated nucleotides can then be removed (e.g., by chemical degradation or by washing) and the next position in the primer-template can be queried with another nucleotide species.